Electrolytic recording medium

ABSTRACT

AN ELECTROLYTIC RECORDING MEDIUM COMPRISING A SHEET OF PAPER IMPREGNATED WITH 25 TO 50% BY WEIGHT OF AN ELECTROLYTICALLY CONDUCTING SOLUTION CONTAINING A MARKING COMPOUND SUCH AS ASCORBIC ACID, ERYTHORBIC ACID OR SODIUM SULFOXYLATE, AND FROM 0.05 TO 25% BY WEIGHT OF AN N-ALKYLOL AMINE-ALDEHYDE ADDITION PRODUCT. SUITABLE ADDITION PRODUCTS ARE METHYLOLUREAS AND HEXAMETHYLOLMELAMINE.

April 20, 1971 DIAMOND ET AL 3,575,821

ELECTROLYTIC RECORDING MEDIUM Filed July 5, 1968 INVENTORS.

ARTHUR S. DIAMOND JAMES F. KOMERSKA United States Patent 3,575,821 ELECTROLYTIC RECORDING MEDIUM Arthur S. Diamond, Palos Verdes Peninsula, and James F. Komerska, Carson, Calif., assignors to Hogan Faximile Corporation, Los Angeles, Calif.

Filed July 5, 1968, Ser. No. 742,644 Int. Cl. B21h 1/20 US. Cl. 2042 13 Claims ABSTRACT OF THE DISCLOSURE An electrolytic recording medium comprising a sheet of paper impregnated with 25 to 50% by weight of an electrolytically conducting solution containing a marking compound such as ascorbic acid, erythorbic acid or sodium sulfoxylate, and from 0.05 to 25% by weight of an N-alkylol amine-aldehyde addition product. Suitable addition products are methylolureas and hexamethylolmelamine.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to electrolytic recording and to an improved elcectrolytic recording medium or paper. More particularly, this invention relates to processes and materials for electrolytic facsimile recording on electrolytically conducting recording media.

(2) Description of the prior art iRecording paper of the electrolytic type is generally marked by passing the paper between a positive eroding metal anode electrode and a negative non-eroding cathode electrode. The paper is impregnated with an electrolytically-conducting solution containing certain ingredients. When a voltage is applied between the electrodes and current is passed through the recording paper, metal ions are introduced into the paper from the anode and reacted with one or more of the ingredients of the paper impregnant to form a colored mark on the recording paper.

To be acceptable for facsimile recording purposes, the recording paper preferably should be white initially and should have the ability to produce marks having a density which is at least close in linear relationship to the current passed through the paper. The density of the marked area should be variable uniformly in a range from white for zero current flow through grays for currents of intermediate magnitude to black for currents of maximum flow. The mark preferably should be black to provide a good contrast with the white background.

There should also be a minimum of bleeding or fringing of the \mark so as to provide good resolution. The unmarked paper While stored in moist condition in sealed containers for extended periods of time prior to use should remain stable without chemical decomposition of its ingredients which would cause discoloration of the paper or affect the reproducibility of results without recorder adjustment. Further, the marked recording paper should not become discolored, give oit unpleasant or other odors, or bs subject to the transfer of the mark to adjacent materials. The marked recording paper should be insensitive to light even after being repeatedly run through any of the conventional copying machines involving exposure to ultraviolet or other strong light sources.

Heretofore successful recording papers have been provided employing pyrocatechin, also called catechol, as the marking compound. While generally satisfactory, some difficulty has been had with the catechol papers because of the tendency of catechol to diffuse or transfer to and to discolor adjacent sheets of paper in contact with or in close proximity thereto. Catechol vapors also may cause discoloration of the walls and other surfaces near the recorder during the recording operation.

Prior to the development of the catechol papers, attempts were made to provide papers utilizing a silver electrode to supply the metal ions which were reacted with a reducing agent to form a mark. Such papers are disclosed in Elsey Pat. No. 2,063,992.. Elsey teaches the use of formaldehyde in an aqueous solution of sodium nitrate and sodium hydroxide to form a mark under alkaline conditions by reduction of the silver ion to metallic silver. Homvever, Elsey states that, although the time factor of reduction to free metal is very small with the solution described, yet during even the time that the silver ion is in solution it may diffuse away from its point of origin and so render the lines less sharp and blur the image when reduction finally occurs. Elsey combats lack of sharpness by adding reagents to react even more quickly, not to reduce the silver ion at once but first to form insoluble compounds with it. With the silver as insoluble compound once fixed to its point of Origin, it is subsequently reduced to free metal. The formulations suggested by Elsey to accomplish his process include polyhydric phenol reducing agents which are difficult to stabilize in alkaline solutions and so, while purportedly stabilized formulations are disclosed, actually the impregnated papers are unstable and discolor within a few days. Further, the free aldehydes of Elsey, in aqueous solution, have high vapor pressures and thus tend to produce objectional odors. Still further, the inability to maintain a fixed aldehyde concentration presents diificulty in obtaining reproduciblility of results.

Ser. No. 6003893, filed Dec. 12, 1966 and now Pat. No. 3,402,109, discloses a non-aldehyde electrolytic recording paper for use in electrolytic recording between a stationary eroding metal anode and a rotatable non-eroding cathode. The medium disclosed in that application contains generally an electrolyte and either ascorbic acid or erythorbic acid as the marking compound and the formulation may also contain one or more buffering agents similar to the paper of Elsey. The paper performs very well when fresh but discolors on the shelf even when stored in heat-sealed polyethylene bags. The moist, impregnated paper will also turn yellow under natural aging conditions within seve al days to a week and on accelerated aging conditions such as ovenkeeping at C., the paper becomes bright yellow within 24 hours. This instability is manifested in the impregnated paper prior to imaging.

SUMMARY OF THE INVENTION An improved electrolytic recording medium is provided according to the invention, said medium overcoming the foregoing difiiculties and disadvantages. In accordance with this invention, a sheet of electrolytic recording medium is impregnated with an electrolyticallyconducting solution which contains a marking compound, an electrolytically-conducting salt and a stabilizing amount of an adduct of an amine and an aldehyde. Preferably, the pH of the solution is about 6 to about 11 and in the preferred form, the medium is buffered in a pH range between 8 and 11.

In a preferred electrolytic recording system according to the invention, the sheet of impregnated electrolytic recording medium is disposed with a silver-containing anode in contact with one part of the sheet and a cathode in contact with another part of the sheet so an electric current can be passed through the electrolyte to affect a color marking reaction where the sheet is contacted by the anode. The silver ions are oxidized and released into the paper by electrolytic dissolution of the anode. These ions are reduced to metallic silver by the action of the marking compound to leave a dense blue-black mark where the paper is contacted by the anode during the flow of electric current. A clear-unblured image is obtained with minimum diffusion of the mark. The recording paper is stable both before and after marking and the recording mark does not fade, even after repeated exposure to light and heat such as is occasioned -by passing the paper through the copying machines which use strong ultraviolet lamps. A distinct advantage of the electrolytic recording mediums of the invention is that the aged brightness retention is excellent, the resulting images having good density and resolution with no tendency for the background to discolor.

These and other aspects of the invention will be more fully understood from the following detailed description when considered in conjunction with the accompanying schematic perspective drawing showing the recording system of this invention.

Any one of a variety of electrolytic recording devices can be used to produce images on the recording medium of this invention. Among those most commonly used for this purpose are the helix-and-bar, single stylus, and stylus-matrix recorders. The former device will serve to describe the operation of the present invention.

Referring now to the drawing, a sheet of paper 10, impregnated with a conductive electrolyte containing a marking compound such as ascorbic acid, erythorbic acid, sodium sulfoxylate, or various mixtures thereof, and a stabilizing amount of an adduct of an amine and an aldehyde is passed between a stationary eroding metal anode 11 and a rotatable drum 12 which has a prismatic platinum-iridium helical wire electrode 13 wound around and projected above its exterior surface. The helical electrode 13 sweeps a spot longitudinally along a knifeedge 14 of the anode electrode bar. Current passing between the electrodes causes the metal anode to oxidize and thereby enter the electrolytic solution of the moist recording medium in the form of metal ions. These ions are quickly reduced to free metal by action of reducing agents, also called marking compounds, in the recording medium which leave a dense blue-black mark where the paper has been contacted simultaneously on opposite sides by the anode and the helical electrode during the =flow of electrical current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The stabilizing agent according to the invention is utilized in the form of an adduct of an amine and an aldehyde. For example, primary or secondary amines such as urea, melamine, hydantoin, imidazolidone or acetamide will react with aldehyde such as formaldehyde to form N-methylol-containing reaction products. For example, urea reacted with formaldehyde forms methylolurea (MU) or dimethylolurea (DMU). The reaction product of melamine and formaldehyde is known as hexamethylolmelamine (HMM), and dimethylhydantoin reacts with formaldehyde to form a product known as methyloldimethylhydantoin or dimethylhydantoin formaldehyde (DMHF). Acetamide and imidazolidone form methylolacetamide and methylolimidazolidone reaction products with formaldehyde. Each of these products has in common the presence of a methylol substituted amino group of the formula N-CH OH. The amine-aldehyde adducts of the invention are present in the formulation in a stabilizing amount of from about percent to 250 percent (dry basis) by weight of the total weight of marking compound, preferably to 180 percent by weight, depending upon the particular marking compounds used.

Formaldehyde-amine adducts are intermediates in the preparation of fully cured aminoplast thermosetting resins and are readily available from several sources, such as American Cyanamid (Parez resins) and Reichhold Chemicals (Beckamine resins).

The preparation of methylolurea, dimethylolurea or hexamethylolmelamine is described in Formaldehyde, Walker, J.-F., ACS Monograph, 3rd edition, Reinhold, New York, 1964. For example, N-methylol-containing adducts of urea or melamine and formaldehyde are obtained by reacting urea or melamine with an excess of formaldehyde with heating and stirring. The excess is preferably from 3 to 10 molar parts of formaldehyde per part of urea or melamine and is preferably from 5 to 8 parts. The mixture is heated to a. temperature of at least about 60 C. to dissolve the reactants but preferably it is heated to a temperature of below C. to prevent excessive cross-linking. The resultant solution contains a preponderance of the N methylol-containing products though it may contain other amine-formaldehyde and other condensation products such as trimethylolmelamine, paraldehyde, etc.

The electrolytic recording medium also contains as a marking compound a reducing agent for the metal ions in a concentration within range of about 1 to 10 percent, wet basis, by weight of the recording medium and more preferably, in an concentration of about 2 to 8 percent by weight. The amount of reducing agent can be varied from a minimal amount which barely reduces enough metal ions to make a visible mark up to the solubility limit of the reducing agent. The required concentration is selected to be compatible with conventional recording speeds and marking apparatus sensitivity. The reducing agent may be selected from ascorbic acid, erythorbic acid or sodium sulfoxylate or various combinations thereof.

As indicated previously, it is preferred to include a substantially neutral strong electrolyte in the composition to provide increased conductivity, especially at higher recording currents and speeds. The electrolyte is preferably a soluble inorganic salts such as alkali metal, or ammonium, nitrates and sulfates which are compatible with the other ingredients, of the composition and do not interfere with the marking reaction. The concentration of the electrolyte salt can be varied as required for changing the electrical conductivity of the solution. Such variations can be from minimal amounts to the solubility limits of the salts used and preferably, they are employed in a concentration within the range of about 2 to 20 percent, wet basis, by weight and more preferably, from about 6 to 12 percent by weight of the impregnating solution.

Various other ingredients such as buffering agents, intensifying agents and image-tone modifying agents may be present. The preferred intensifying agents are alkali metal salts of stannates, silicates or aluminates. The sodium and potassium stannates are particularly preferred since they provide better stability on long storage. These intensifying agents are believed to serve as nucleating agents, especially with silver marking anodes, by providing sites upon which metallic silver grains can be deposited.

The pH of the medium can be adjusted an buffered to any desired value by well-known buffering agents. Formulations can be made to work satisfactorily in the acid or alkaline range, but those with a pH in the vicinity of 8 to 11 give the best overall results wtih respect to clarity, density of the mark, and stability of the recorded sheet. The pH of any of the above solutions can be buffered as high as 11.0 by including within the solution buffering agents such as alkali metal carbonates, borates, phosphates, or silicates. Potassium carbonate is preferred in ascorbic acid-containing compositions since potassium ions have been found to impart greater density to silver images and provide a more neutral black color. Another reason for pH adjustment is to accommodate base papers having various levels of acidity.

Alkali metal salts of lower alkanoic acid such as sodium formate or sodium acetate can be added to modify image tone to favor blue-black recordings rather than brownish colored reproductions. Either of these can be used alone or in combination as image tone modifiers. A

metal chelating agent such as ethylenediaminetetraacetic acid (EDTA) is a useful additive in tying up metal ions to improve stability of ascorbic or erythorbic acid-containing solutions. It should be noted, however, that EDTA affects image color adversely, giving it a distinct brownish tone. Various anionic, cationic or non-ionic dispersing agents, such as the Triton (Rohm and Haas Co., Philadelphia, Pa.) surfactants which are based on alkylaryl polyether alcohols, sulfonates and sulfates are useful adidtions to the impregnating solution as they asure uniform distribution of liquid into the paper stock.

To prepare the impregnating solution, the ingredients are dissolved in water with moderate heating if required to eifect solution. Preferably, the nitrate or surfate electrolyte is dissolved first with the successive addition of the ascorbic acid, erythorbic acid or sodium sulfoxy-late marking ingredient and then the nucleating agent, buffering agents and tone-modifying agents, added in that order, with the amine-aldehyde adduct added last. The carbonate should be added slowly as etfervescence does occur with the first two named marking ingredients.

A recording medium according to the invention is prepared by impregnating porous white paper or other support with an impregnant prepared as discussed above. High wet-strength papers are normally used to avoid web breakage on roll-fed electrolytic recorders. The wetstrength resin can be either an acid or alkaline type, but the latter is much preferred as it provides better image color and improved shelf life. The impregnant is permitted to distribute itself evenly throughout the paper. Excess impregnant is removed by passing the paper between pressure rollers, so that when the paper is ready for use in a recorder, the impregnated paper contains about 25 to 50 percent moisture by weight and preferably, 35 to 42 percent by Weight on a wet basis. To retard evaporation and maintain the moisture content of the paper substantially uniform, the impregnated paper is stored in a suitable sealed container such as a heatsealed polyethylene bag until ready for use.

The invention will now be illustrated in the following detailed examples. It is understood that these examples are offered by way of illustration only and are not intended in any way to limit the invention.

EXAMPLE I Preparation of HMM Sixty-five ml. of 37% formaldehyde (0.8 mol CH O) was measured and poured into a 250 ml. beaker. Ten ml. of demineralized water was added. The beaker was placed on a hot plate and heating was initiated. 12.6 gms. of melamine powder was added to the heated formaldehyde solution with vigorous stirring. The mixture was heated to 75 C. for about minutes or until all the melamine dissolved and the solution cleared. The solution was then rapidly cooled to room temperature. The solids content of the resultant mixture is approximately 44 percent.

EXAMPLE Il Preparation of DMU Seventy-six ml. of 37% formaldehyde was measured into a 250 ml. beaker. The pH of the formaldehyde solution was adjusted to a pH of about 8.0 with 1.0 N NaOH. The solution was placed on a hot plate, heated to 80 C., and then 10 gms. of urea was added with stirring. Heating at 8090 C. was continued for about minutes and the solution was then cooled to room temperature. Final solution volume after cooling to room temperature is about 78.5 ml. and the mixture contains about 52 percent solids.

6 EXAMPLE 111 Ascorbic acid A masterbatch containing the following ingredients was prepared:

The masterbatch was formulated with varying amounts of HMM and DMU as follows:

A B C D E Master-batch, milliliters 100 100 100 100 H VIM (Ex. 1), milliliters 0.5 2.0 5.0 20. 0 5 M K2003 2. 5 2. 5 2. 5 2. 6 2. 5

Forty mls. of each formulation were impregnated into 8%" X 30 handsheets by pouring the solution into a polyethylene bag containing the dry, folded sheet, allowing several minutes for uniform wetting and then expressing excess solution from the bag by means of a handoperated clothes wringer. The handsheets were tested on a silver blade facsimile marking Quickfax 900 machine (manufactured by Telautograph Corporation). As compared to sample A (no HMM), the density of the mark was of substantially the same quality with progressive decrease in the brownish cast as the amount of HMM increased. Samples B through E all showed a much whiter background than Sample A. Samples .A through E were heat-aged overnight at 65 C. Sample A turned yellowbrown, Samples B through E were progressively lighter, and Sample :15. was the Whitest of the series.

EXAMPLE IV A 1000 ml. masterbatch was prepared in accordance with Example III above to which was added 20 ml. of HMM (8.8 grams solids). This solution was dip coated onto Crane type 33-91 alkaline wet strength paper at speeds of 30, 40 and 50 feet per minute and then squeegeed between a pair of rubber rollers loaded at 3.5 to 4.5 pounds per linear inch pressure, giving a moisture content of from 36 to 39 percent. Better printing densities were achieved as the moisture content increased.

EXAMPLE V Sodium sulfoxylate-DMU A masterbatch containing the following ingredients was formulated by combining the ingredients in the order listed:

Demineralized H O ml. 1600 KNO gms 192 Na stannate gms 4 K CO gms 10.2 Triton X-100 (10%) ml 20.8

Example A B C D E Mast-erbateh, milliliters 100 100 100 100 100 Na3Sz04, grams 4 4 4 6 6 DMU (Ex. 2). milliliters 2 4 8 2 8 pH (final) 7. 7 s. s 10.3 7. 2 7. 4 Initial print P F Ex. U G Aged print P G F U P All samples were impregnated into 8 /2" x 30" sheets of Crane 33-91 paper on the basis of 50 ml./sheet and were squeegeed to a 40 percent moisture content. Prints were prepared on a silver blade recorder and were evaluated on the basis: Ex (excellent), G (good), F

7 (fair), P (poor) and U (unsatisfactory). All aged prints had minimal background discoloration. The DMU solution contained about 52 percent solids. On this basis, it was found that the weight ratio of DMU to Na S O should be at least 0.70 to obtain a satisfactory print, and within a range of DMU to Na S O of about .50 to about 1.05.

EXAMPLE VI Sodium sulfoxylate-HMM A masterbatch containing the following ingredients was formulated by combining the ingredients in the order listed:

Fifty mls. of each solution was impregnated into an 8 /2 x 30" sheet of Crane 3391 paper base and squeegeed to 40 percent moisture. At the higher pH the solutions were less cloudy. Sample D showed the best initial and final printing characteristics, printing with an excellent black mark on a white background. The weight ratio of HMM (about 44 percent solids) to sulfoxylate of about 1.76, dry basis, and a pH of -10.6 appeared best for a black mark.

EXAMPLE VII Sodium sulfoxylate-ascorbic acid Electrolytic recording media containing the following ingredients were prepared:

Demineralizod H2O, milliliters. 100 100 KNOn, grams 6 6 L-aseorbic acid, grams... 6 6 Na sulloxylate, grams. 2 2 HMM (Example 1), milliliters 8 DMU (Example 2), milliliters 4 Sodium stannate, grams 0.2 0. 2 Sodium iormate, grams 1.0 1.0 Triton X-100, (10%) millliters 1.0 1. 0

Fifty ml. of sample solutions VII-A and VII-B were impregnated into 8 /2 x 30" sheets of Crane 339l. Excellent resolution, blue-black prints on a white background were obtained in each case. Very little change in print quality was observed with aged samples.

Commercial runs of recording medium are prepared by impregnating white, wet strength paper or other porous, absorbent high wet strength support with an impregnant prepared in accordance with the instant disclosure. The alkaline type of wet strength papers are preferred. These papers usually contain as a wet strength agent a minor amount, below 1% by weight based on the paper, of a cationic cross-linked linear polyamide. Typically, a polyamide prepared by reacting a saturated aliphatic dicarboxylic acid with a polyalkylene-polyamine is cross-linked by reaction with epichlorohydrin. Acid wet strength papers containing about 1% by weight of a cross-linked, acid cured melamine-formaldehyde resin may also be utilized but the resulting electrolytic recording medium is not as stable.

The impregnant is allowed to distribute itself evenly throughout the paper. Excess impregnant is removed by passing the paper between pressure rollers so that when ready for use in a recorder, the impregnated paper has about 25 to 50 percent moisture by weight. To retard evaporation and to maintain the moisture content of the paper substantially uniform, the impregnated paper is stored in a suitable moisture-proof sealed container until needed for recording.

The invention thus provides a process and medium which may be employed with all the available electrolytic recording machines over the entire range of speed and marking currents. The use of the aminealdehyde adducts has been found to stabilize the paper and to provide prints having a superior background whiteness and excellent tonal quality and resolution is achieved in formulations including ascorbic acid, erythorbic acid, sodium sulfoxylate or combinations thereof.

It will be apparent to those skilled in the art that numerous substitutions, alterations, and modifications are permissible without departing from the spirit or scope of the invention as defined in the following claims.

What is claimed is:

1. An electrolytic recording medium for forming a record by reducing electrolytically dissolved silver ions provided from a silver containing anode comprising a sheet impregnated with an electrolytically conducting solution containing 1 to 10% by weight of the medium of at least one marking compound selected from the group consisting of ascorbic acid, erythorbic acid and sodium sulfoxylate and a stabilizing amount of from 0.05 to 25% by weight of an N-lower alkylol addition product of an aldehyde of the formula ZCHO where Z is selected from the group consisting of hydrogen and alkyl of 1 to 10 carbon atoms and an amine selected from the group consisting of melamine, urea, hydantoin and imidazolidone.

2. A medium according to claim 1 wherein said impregnated sheet contains between about 25 and about 50% of moisture of weight of said sheet.

3. A medium according to claim 2 further including a sealed container surrounding said sheet.

4. A recording medium according to claim 1 in which the marking compound is ascorbic acid.

5. A recording medium according to claim 1 in which the marking compound is erythorbic acid.

6. A recording medium according to claim 1 in which the marking compound is sodium sulfoxylate.

7. A recording medium according to claim 1 in which the marking compound is a combination of sodium sulfoxylate and ascorbic acid.

8. A recording medium according to claim 1 in which the marking compound is a combination of sodium sulfoxylate and erythorbic acid.

9. A recording medium according to claim 1 in which the pH of the medium is from about 6 to 11.

10. A recording medium according to claim -1 containing 2 to 20% by weight of an electrolytically conducting salt selected from the group consisting of alkali metal and ammonium nitrates and sulfates.

11. A recording medium according to claim 1 in which the addition product is hexamethylolmelamine.

12. A recording medium according to claim 1 in which the addition product is methylolurea.

13. A method of recording comprising contacting a sheet impregnated with an electrolytically-conducting marking solution with a silver metal containing anode on one side of the sheet and a cathode in contact with the obverse side of the sheet and passing an electric current therethrough to produce a colored mark on the recording medium; said solution containing 1 to 10% by weight of the medium of at least one silver marking compound selected from the group consisting of ascorbic acid, erythorbic acid and sodium sulfoxylate and a stabilizing amount of from 0.05 to 25% by weight of an N-lower alkylol addition product of an amine selected from the group consisting of melamine, urea, hydantoin and imidazolidone and an aldehyde of the formula ZCHO where Z is selected from the group consisting of hydrogen and alkyl of 1 to 10 carbon atoms.

1 0 References Cited UNITED STATES PATENTS TA-HSUNG TUNG, Primary Examiner T. TUFARIELLO, Assistant Examiner 

